2017 年 35 巻 p. 39-45
It is well known that methane hydrate exhibits abnormal stability, so-called “self-preservation effect” at temperatures of 240 K to 270 K and atmospheric pressure, though the equilibrium temperature of methane hydrate at atmospheric pressure is approximately 190 K. The ice shielding at the surface of methane hydrate would be one of the most important steps toward developing the self-preservation. That is, to observe the phase and morphology changes from methane hydrate to ice is significant. We have observed the microstructural change of the synthetic methane hydrate during its decomposition at the temperatures of 263 K and 293 K with a combination of scanning electron microscopy (SEM) and the freeze-fracture replica method. The SEM images reveal that the methane hydrate crystal has a structure arranging the clusters of 20 nm in diameter. When the methane hydrate is partially decomposed during taken from the high-pressure cell (rapid depressurization at 253 K), a part of the clusters changes to the cluster aggregates of 60-200 nm. The cluster aggregates gradually grow from their peripheries to the hexagonal ice crystals during gradual decomposition at 263 K. The microstructural change supports the decomposition mechanism of methane hydrate by ice-shielding under a temperature condition with self-preservation effect. At 293 K, the methane hydrate is immediately decomposed. The residual aqueous solution after complete decomposition contains the large number of ultrafine bubbles (nanobubbles) of 100 nm or less in diameter.